It was suggested in a recent article from our laboratory that most enzymes in vivo probably do not respond according to Michaelis-Menten theory. These conclusions were alluded to from computer simulation studies using initial rate equations for a variety of non-allosteric kinetic mechanisms. In these simulations the computer was programmed with theoretical rate equations, kinetic parameters and substrate and product concentrations. We plan to carry out experiments with enzyme systems in vitro using reported levels of intracellular substrates and products in order to evaluate enzyme responses in a near in vivo environment. These studies should serve either to support or refute our computer simulation findings. The enzymes that are to be used in these experiments include brain and muscle hexokinase, muscle creatine kinase and liver lactate dehydrogenase. These enzyme studies may provide information on whether enzymes have the inherent ability of self regulation which is a consequence of kinetic mechanism.